provider/openstack: Don't update volume tags when storage is unsupported

[babbageclunk]

Description of change

In nova-lxd, storage isn't supported, so ownership transfer would fail during migration when we tried to get volumes to update their controller tags. Detect the not-supported error and skip updating volumes in that case.

QA steps

• Bootstrap 2 controllers (A and B) on a nova-lxd Openstack cloud.
• Add a model m to A.
• Deploy a minimal charm to m.
• Migrate m from A to B.
• The migration (and in particular the ownership transfer) should complete successfully.
• Destroy controller A.
• The model m (now running under controller B) should be unaffected.

Bug reference

Fixes https://bugs.launchpad.net/juju/+bug/1677225

[babbageclunk]

$$merge$$

[jujubot]

Status: merge request accepted. Url: http://juju-ci.vapour.ws:8080/job/github-merge-juju